The present invention relates to a projection type image display apparatus of the type which is used as a projection TV set, etc., in which white light from a light source is separated into additive primary colors, each of the primary colors is modulated with the use of a display element, and images are expanded and displayed on a screen.
Along with a diversity of video sources, projection type image display apparatuses are popular as optical projection apparatuses for a large screen as a result of its marketable properties, such as lightness in weight, low price, and compactness in size. In particular, the projection type image display apparatuses using a liquid crystal display element (hereafter, referred to as a liquid crystal panel) as a video generation source has come onto the market because of recent significant improvement of the definition and numerical aperture of a liquid crystal panel. Unlike the conventional projection type CRT, the liquid crystal panel does not emit light by itself, so it needs a light source. The projection type image display apparatus with a liquid crystal panel is composed so that a white light from its white light source is separated into additive primary colors and each of those primary colors are modulated in the liquid crystal panel, from which full-color images are displayed on the screen by expanding original images on the liquid crystal panel through a projection lens unit.
The optical system of the projection type image display apparatus that employs this liquid crystal panel is divided into two types, i.e. a three-panel type that uses three liquid crystal panels and a single-panel type that uses only one liquid crystal panel.
The three-panel type optical system has a liquid crystal panel and an optical unit (color separator) for each respective color of the primary colors (red, green, and blue) obtained by separating white light. The optical unit (color separator) propagates one of the obtained primary colors and the liquid crystal panel modulates the intensity of the colored light to form an image. Each color image is superposed with the other color images optically (color synthesizer) so as to display an image in full colors. This three-panel configuration of the optical system has advantages in that the light from the white light source can be used effectively to obtain high purity colors. In spite of this, because the optical system requires both a color separator and a color synthesizer as described above, the number of parts is increased in the optical system and, accordingly, the cost becomes higher than that of the single-panel configuration.
On the other hand, the single-panel configuration of the optical system uses only one liquid crystal panel, and it is divided into two types according to how TFT apertures are disposed in itself; delta type and stripe type. In the early single-panel configuration, a color filter was used to separate a white color into additive primary colors, but the configuration was plagued with the problem in practical use that the color filter absorbed and reflected the light, thereby the usage efficiency of the light was lowered to about ⅓ that of the three-panel configuration.
In order to solve this problem, for example, the Japanese Patent Unexamined Publication No. 4-60538 has disclosed a single-panel color liquid crystal display apparatus, which, as shown in FIG. 1 thereof, employs dichroic mirrors 4R, 4G, and 4B disposed in a fan-like pattern so as to separate white color light obtained from a white color light source 1 into red, green, and blue light fluxes, thereby improving the usage efficiency of the light.
In this apparatus, each of the light fluxes R, G, and B separated by the above dichroic mirrors 4R, 4G, and 4B is injected at a different angle from the others into a micro-lens array 10 disposed at the light source side of a liquid crystal display element 20 shown in FIG. 2 in the above-referenced publication.
Each light flux passing this micro-lens array 10 is distributed and irradiated at a liquid crystal site driven by a signal electrode to which a color signal corresponding to one of those light fluxes is applied. Consequently, the usage efficiency of the light is greatly improved, thereby obtaining brighter images than the liquid crystal display element that employs an absorption type color filter.
The official gazette of Japanese Patent Laid-Open No. 5-328805 has also disclosed a projection type color liquid crystal display apparatus that has improved color purity by minimizing the generation of stray lights by starting the separation of white color light into the additive primary colors at the long wavelength side so as to prevent color mixing caused by the angle dependency of the wavelength selection characteristics of each of the dichroic mirrors. According to this method, because the original light is separated into light fluxes in the order of R, G, and B, thereby shifting the characteristics of each dichroic mirror, stray lights are not generated easily and the color purity of each separated light flux is improved. Images can thus be projected at a wide range of color reproduction.
However, in the technique disclosed in the above-referenced publication where the angle α is obtained when the G light flux is injected at an angle close to the normal of the liquid crystal display element, as shown in FIG. 6(a) thereof, and is diffracted by a micro-lens and the angle β is obtained when each of the R and B light fluxes is injected obliquely to the normal of the liquid crystal display element, as shown in FIG. 6(b) thereof, and is diffracted by a micro-lens; the angle β is larger than the angle α of the light flux (G) irradiated from the liquid crystal display element. This requires a large diameter (low F value) projection lens, thus becoming a primary factor for increasing the manufacturing cost of the projection type color display apparatus.
In order to solve this problem, the Japanese Patent Unexamined Publication No. 8-114780 disclosed a method for keeping a favorable white balance with the use of a small diameter projection lens by injecting a color light emitted from the light source with the weakest spectrum at an angle close to the normal of the liquid crystal display element, thereby eliminating the eclipse at the pupil of the projection lens with the least volume color light.
Because the purity of the color light with the least light volume is improved, it is possible to obtain a wider color reproduction range and more clear images. One of the projection lenses used for the optical system of the projection type image display apparatus described above is a retrofocus lens of the type disclosed, for example, in the Japanese Patent Unexamined Publication No. 9-96759. (Because of the long flange back, it is the most suitable for the three-panel configuration of the optical system.) Because the half-angle of view of this projection lens is about 42°, the projection distance is short. If it is employed for a back-projection type image display apparatus, therefore, the arrangement will be more compact in size even when only one reflection mirror is employed.
Generally, the transmission type screen used in this case employs a two-panel configuration consisting of a lenticular sheet and a Fresnel lens sheet. In some cases, the transmission type screen is also provided with a lenticular lens on the image light injection surface of the Fresnel lens sheet so that the lenticular lens is shaped so as to be longer in the horizontal direction of the screen.
However, in the single-panel configuration described above it is difficult to obtain a predetermined purity for each color. Only with the means proposed in the Japanese Patent Unexamined Publication No. 8-114780. This is because, according to this method, each of the R, G, and B light fluxes separated by a dichroic mirror is injected at a different angle from the others into the micro-lens array 7 disposed at the light source side of the liquid crystal display element shown in FIG. 7 thereof. Each light flux passing this micro-lens array 7 is distributed and irradiated on the liquid crystal sites 24G, 24R, and 24B driven by a signal electrode respectively to which a color signal corresponding to each color light flux is applied independently. At this time, each junction between those micro-lenses provided at the micro-lens array 7 is not formed sharply, thereby it disperses the light. Consequently, for example, part of the green light flux, whose relative visibility is the highest and whose emission spectrum from the light source is dispersed at the junction, is then mixed into the red light flux whose emission spectrum from the light source is the weakest. Thus, the red color purity is lowered at the liquid crystal site 24R due to the mixture of the red light flux and the green light flux. The liquid crystal site 24R is originally injected only with the red light. This is why each color purity cannot reach its predetermined value with the above method.
If the reflection characteristics of the dichroic mirror for separating the red color are set so as to improve the purity thereof, however, the light volume of the red light flux to be obtained is reduced, thereby the white balance obtained by adding the three primary colors is lost.
At this time, if the white balance is adjusted by reducing the light volume of each of the other two color lights, then the luminance of the white video obtained by adding the three primary colors is lowered.
As described above, even in the case of the projection type color liquid crystal display apparatus proposed in the Japanese Patent Unexamined Publication No. 8-114780, both the brightness and the color purity are not able to reach satisfactory levels when compared with those of the projection type display apparatus that employs a conventional projection type CRT. In addition, because the luminance level is high when images are displayed in black on the liquid crystal panel, the contrast of the images becomes unfavorably low.
On the other hand, in order to realize a compact rear projection type image display apparatus for general home use, the projection distance (distance between the projection lens unit and the screen) must be reduced. Thus, a wider projection lens unit is required. At this time, if an ordinary wide projection lens unit is used for the apparatus, the peripheral light volume ratio is reduced significantly due to the light distribution characteristics of the liquid crystal panel. This is because the spectrum transmittance and reflectance of each of the three dichroic mirrors disposed between the liquid crystal panel and the white light source differs among injection angles of the light, so that the light flux from the white light source is injected into each dichroic mirror and the liquid crystal panel. As a result, the main light beam injected into the projection lens unit from each object point of the liquid crystal panel goes approximately in parallel to the light axis of the projection lens unit and the distributed angle becomes proportional to the numerical aperture of the micro-lens. If a wider projection lens is employed for the optical system, then the light fluxes to be injected into the projection lens unit from around the liquid crystal panel is reduced extremely, thereby the peripheral portion of each expanded image on the screen becomes dark.
In addition to the problems described above, the above-mentioned method is also confronted with the following problems that must be solved. (1) Each image must be focused accurately in every corner. (The chromatic aberration of the magnification must also be reduced.) (2) The F value must be reduced so as to improve the brightness of the screen. (3) Because of the inability of convergence adjustment, the distortion must be reduced. (4) The reflection on the lens surface must be reduced, to the extent of suppressing the loss of brightness and securing the contrast property sufficiently.
As described above, the projection lens units proposed to data have many problems that must be solved. Actually, however, even the retrofocus lens proposed in the Japanese Patent Unexamined Publication No. 9-96759 cannot secure enough brightness because of the large F value (2.56) and the shorter projection distance while the half-angle of view is about 42°.
The conventional optical projection system that employs a liquid crystal panel is also provided with a normal white light source and a cooling fan for cooling the liquid crystal panel (including a polarizing plate). Consequently, the cost of the optical system is increased and the reduction of the blowing sound has been a problem that must be solved. In the case of the air-cooling method, it is difficult to cool down the polarizing plate satisfactorily. The polarizing plate is thus affected by the heat and experiences a change in physical properties, thereby deteriorating the polarization degree and the contrast.
On the other hand, the transmission type screen used for the apparatus is manufactured by the conventional technique proposed in the Japanese Patent Unexamined Publication No. 58-59436. According to the conventional technique, the lenticular lens disposed on the injection surface is part of an elliptic cylindrical surface and the ellipse is formed so that the long axis is assumed in the direction of thickness between the injection surface and the ejection surface, and one of the two focal points of the ellipse is positioned inside the substrate and the other focal point is positioned around the ejection surface. In addition, the eccentricity of the ellipse is selected so as to take an approximate inverse number of the refractivity of the base material.
As a result, if a light flux in parallel to the long axis of the ellipse is injected in the injection surface, the light beam goes into aberration entirely at the focus around the ejection surface, causing the light beam to be dispersed from this focal point in the horizontal direction of the screen.
On the other hand, the lenticular lens provided on the ejection surface has an elliptic cylindrical surface formed almost symmetrical to the elliptic cylinder on the injection surface. The actual lenticular lens sheet does not cause the light to be focused at a point, but is dispersed, since a dispersion material is mixed in the lens sheet, as shown in FIGS. 31 and 32 thereof. Consequently, it is impossible to increase the width of the light absorption layer in the horizontal direction of the screen by more than the width of the lenticular lens. The reflected light caused by an external light cannot be reduced and the reduction of the contrast cannot be suppressed within a fixed value.